1. Field of the Invention
The present invention relates to a charge pump drive circuit and, more particularly, to a charge pump drive circuit for a light emitting diode.
2. Description of the Related Art
FIG. 1(a) is a detailed circuit diagram showing a conventional charge pump drive circuit 10. The charge pump drive circuit 10 converts an input voltage source Vin into a drive voltage Vout for driving a load 11. Sometimes the input voltage source Vin is not at an appropriate status to be directly applied for driving the load 11, such as in the case where the input voltage source Vin is too high, too low, or fluctuating very much. For this reason, the charge pump drive circuit 10 is necessary for generating an applicable and stable drive voltage Vout through regulating the input voltage source Vin. For example, the conventional charge pump drive circuit 10 is provided with a 1:2 step-up charge pump 12, which is operated alternately between a charging phase and a discharging phase in accordance with switch control signals SC1 and SC2 generated from a switch control circuit 13, thereby generating a drive voltage Vout that is twice as large as the input voltage source Vin.
More specifically, the 1:2 step-up charge pump 12 shown in FIG. 1(a) consists of a first switch S1, a second switch S2, a third switch S3, a fourth switch S4, and a pumping capacitor Cp. The first switch S1 is coupled between the input voltage source Vin and a first electrode of the pumping capacitor Cp; the second switch S2 is coupled between the input voltage source Vin and a second electrode of the pumping capacitor Cp; the third switch S3 is coupled between the second electrode of the pumping capacitor Cp and a ground potential; and the fourth switch S4 is coupled between the first electrode of the pumping capacitor Cp and the drive voltage Vout. As shown in FIG. 1(b), the first and second switch control signals SC1 and SC2 generated from the switch control circuit 13 are non-overlapping with respect to each other, each of which is a binary oscillating signal having a high level H and a low level L. The first switch control signal SC1 is applied to the first and third switches S1 and S3 while the second switch control signal SC2 is applied to the second and fourth switches S2 and S4. During the charging phase, such as from time T1 to time T2 and from time T5 to T6, the first and third switches S1 and S3 are turned ON while the second and fourth switches S2 and S4 are turned OFF, causing the first electrode of the pumping capacitor Cp to be connected to the input voltage source Vin and the second electrode of the pumping capacitor Cp to be connected to the ground potential. During the discharging phase, such as from time T3 to time T4 and from time T7 to time T8, the second and fourth switches S2 and S4 are turned ON while the first and third switches S1 and S3 are turned OFF, causing the first electrode of the pumping capacitor Cp to be connected to the drive voltage Vout and the second electrode of the pumping capacitor Cp to be connected to the input voltage source Vin. Through the alternate operations between the charging and discharging phases, the pumping capacitor Cp is able to provide the drive voltage Vout that is twice as large as the input voltage source Vin.
In order to maintain the drive voltage Vout at the desired regulation value, the conventional charge pump drive circuit 10 is further provided with a feedback control system including a voltage detection circuit 14, an error amplifier 15, a reference voltage source Vref, and a variable resistance unit VAR. The voltage detection circuit 14 is directly coupled to the output terminal of the 1:2 step-up charge pump 12 for generating a feedback signal Vfb representative of the drive voltage Vout. As shown in FIG. 1(a), the voltage detection circuit 14 may be implemented by a resistive voltage divider in which the coupling point between the series-connected resistors R1 and R2 is used for providing a partial voltage of [R2/(R1+R2)]*Vout as the feedback signal Vfb. Based on a difference between the feedback signal Vfb and the reference voltage source Vref, the error amplifier 15 generates an error signal Verr for controlling the variable resistance unit VAR. Since the variable resistance unit VAR provides a variable resistance between the input voltage source Vin and the pumping capacitor Cp, the charging current into the pumping capacitor Cp during the charging phase may be adjusted and the discharging current out of the pumping capacitor Cp during the discharging phase may be adjusted, thereby effectively regulating the drive voltage Vout provided by the pumping capacitor Cp. As a result, when reaching at the stable state the feedback signal Vfb is regulated to become substantially equal to the reference voltage source Vref, and at the same time the drive voltage Vout is correspondingly regulated as expected. Finally through a typical filter 16 for removing some possible ripples, a desired drive voltage Vout is applied to the load 11.
Although the conventional charge pump drive circuit 10 shown in FIG. 1(a) is able to provide a stable, regulated drive voltage Vout to the load 11, the conventional charge pump drive circuit 10 finds itself incompetent to control the brightness of a light emitting diode that is used as the load 11 because the brightness of the light emitting diode is determined by the drive current rather than the drive voltage. Moreover, in order to ensure that the light emitting diode is actually turned on in any case, the conventional charge pump drive circuit 10 is typically designed to supply a higher-than-necessary drive voltage Vout at the cost of reducing the driving efficiency.